Inasmuch as the rate of dissolution of a particle can increase with increasing surface area, i.e., decreasing particle size, efforts have been made to control the size and size range of drug particles in pharmaceutical compositions by a variety of methods, including various milling techniques, such as airjet milling and wet milling. However, there tends to be a bias in the pharmaceutical arts against milling techniques, particularly, wet milling, due to concerns associated with contamination. For example, in the preparation of pharmaceuticals for oral and parenteral applications, it is desirable to have total contamination, e.g., of heavy metals, below about 10 parts per million. The need to control and minimize contamination is particularly critical in the milling of parenteral products due to potential safety issues associated with injection of contaminants.
Various grinding media, such as stainless steel, zirconium silicate, zirconium oxide, glass, and the like, typically in the form of spherical beads, are commonly used in various mills, including media mills, for grinding materials. However, the use of stainless steel media can result in the introduction of iron, chromium and/or nickel contamination to the milled product accompanied by product discoloration. Media fabricated of conventional materials, such as zirconium silicates and zirconium oxides often contain zirconium, silicon, barium, lead, hafnium, yttrium, thorium and uranium, all of which can enter the product during grinding, leading to potential safety issues. Glass media can contain various alkali oxides, which are an unacceptable source of contamination. Additionally, most commercially available glass media for fine grinding are of the soda lime type, which is not well suited for milling pH sensitive products due to high alkalinity which can result during milling.
Liversidge et al, U.S. Pat. No. 5,145,684, and EPO 498,492, describe dispersible particles consisting of a drug substance or an x-ray contrast agent having a surface modifier adsorbed on the surface thereof in an amount sufficient to maintain an effective average particle size of less than about 400 nm. The particles are prepared by dispersing a drug substance or contrast agent in a liquid dispersion medium and wet grinding in the presence of rigid grinding media. Particles free of unacceptable contamination have been prepared in accordance with this method.
Nevertheless, further reduced levels of contamination are desired. This is particularly so when 1) the drug substance or imaging agent is to be ground in a high energy mill where contamination tends to be particularly problematic, and/or 2) the drug substance or imaging agent is intended for parenteral use, in which case the risks associated with contaminated product can be particularly severe.